This patent application claims priority under 35 U.S.C. xc2xa7365 from International Application Serial No. PCT/GB98/03643, filed Dec. 7, 1998, which claims priority from United Kingdom Application Serial No. 972711.2, filed Dec. 30, 1997, and United Kingdom Application Serial No. 9800144.9, filed on Jan. 7, 1998, which applications are incorporated herein by reference for all purposes.
The present invention relates to a method for inserting an elongate, flexible light transmitting member into a tube, and relates particularly, but not exclusively, to a method for inserting one or more optical fibres into a tube.
EP 0108590 discloses a method of installing an optical fibre member into a duct by causing air flow along the duct such that the fluid drag between the flowing air and the optical fibre member moves the fibre member along the duct. The fibre member is pushed into the duct by means of drive wheels until the surface area of the fibre member exposed to the air flow is sufficiently large to enable the fluid drag to pull the fibre member along the duct to install it in the duct.
The drive wheels are used to address problems caused by the compressibility of air entering the duct. Because of the compressibility of air, the pressure difference necessary between the ends of the duct to install a long length of fibre into the duct causes the flow rate of the air at the input end of the duct to be considerably less than at the output end. As a result, the fluid drag at the input end is often insufficient to draw the fibre member into the duct. Without the drive wheels, the air pressure may even cause the fibre member to move out of the input end of the duct. The drive wheels push the fibre into the duct until a sufficient length of fibre is in the duct to enable the fluid drag to cause the fibre to move along the duct.
However, the extent to which this problem can be overcome with this prior art method is limited, since the optical fibre has a tendency to buckle in the duct, with the result that the fibre may become wedged. It is therefore necessary to employ sophisticated electronic devices to detect buckling of the fibre and stop the drive mechanism in response to this. In addition, it is extremely difficult to install fibre into a longer duct comprising a series of lengths of duct by arranging such prior art devices in series such that each device blows the fibre along a length of duct corresponding to the maximum distance along which the fibre can be blown by a single device. This problem mainly arises because it is extremely difficult to coordinate the installation speed of two such devices which may be separated by up to 2000 meters, as a result of which damage to the fibre may arise.
JP-A-63-124005 discloses a method of installing a reinforced fibre optic cable into a duct by attaching tensile force fibres to the forward end of the fibre optic cable, and blowing the cable into the duct while simultaneously pulling the tensile force fibres from the opposite end of the duct. However, this arrangement suffers from the drawback that if any bends occur in the duct (a frequent occurrence in practical arrangements), applying tension to the tensile force fibres and the fibre optic cable brings the fibres and the cable into contact with the inner wall of the duct, thus causing a frictional force between the tensile force fibres and/or the fibre optic cable and the wall of the duct. This can often greatly increase the tensile force needed to be applied to the tensile force fibres, which can easily damage the fibre optic cable unless the cable has substantial tensile reinforcement. As a result, this arrangement is wholly unsuitable for use with unreinforced or lightly reinforced fibre optic cables.
Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.
According to an aspect of the present invention, there is provided a method of inserting an elongate, flexible, light transmitting member into an elongate tube, the method comprising:
inserting an elongate, flexible, tensile member into the tube;
inserting an elongate, flexible, tensile member into the tube;
attaching the tensile member adjacent a rearward end thereof to the light transmitting member adjacent a forward end thereof;
introducing the forward end of the light transmitting member into the tube; and
causing flow of fluid along the tube to cause the tensile member to move along the tube and the light transmitting member to move into the tube, wherein the tensile member in use, is caused to move along the tube solely by said flow of fluid.
By attaching a tensile member, the other end of which is not pulled, to the forward end of the light transmitting member, the fluid flow along the tube tends to locate the tensile member towards the axial centre of the tube. The fluid drag between the moving fluid and the tensile member causes the tensile member to move along the tube, but minimises the tendency of the tensile member to come into contact with the walls of the tube. This in turn applies a forward force to the forward end of the light transmitting member, which assists the fluid drag between the fluid and the light transmitting member in moving the light transmitting member along the tube, but without the necessity of applying excessive tension to the tensile member which may otherwise damage the light transmitting member. This provides the advantage of enabling the method to be used to install unreinforced or lightly reinforced fibre optic members. The method also has the advantage of enabling significantly longer installation distances than are achievable with prior art methods.
The fluid preferably comprises air and/or pressurised gas.
Alternatively, the fluid may be a liquid, for example water.
In a preferred embodiment, the tensile member is inserted into the tube prior to positioning of the tube at a location at which the light transmitting member is to be inserted therein.
This provides the advantage of avoiding the necessity of installing the tensile member in the tube on site, i.e. at the location at which the light transmitting member is to be installed.
The tensile member is preferably installed in the tube during manufacture thereof.
The tensile member may be attached at one end thereof to a plug adapted to fit in an end of the tube.
This provides the advantage of facilitating location of the end of the tensile member prior to use.
The forward end of the light transmitting member may be introduced into the tube by means of control means for controlling the speed of the light transmitting member.
The forward end of the light transmitting member may be introduced into the tube by means of a drive mechanism.
This provides the advantage of enabling control of the rate of installation of the light transmitting member by virtue of setting the drive mechanism to introduce the light transmitting member at a lower rate than would otherwise occur as a result of fluid drag alone.
The method preferably further comprises the step of controlling the speed of the light transmitting member in response to slackness in the light transmitting member.
This provides the advantage of avoiding situations in which the light transmitting member is fed into the tube faster than the rate at which the fluid drag is carrying the member along the tube, which would cause the light transmitting member to buckle and become damaged.
The tensile member preferably has a relatively low coefficient of friction with the internal wall of the tube.
In a preferred embodiment, the tensile member comprises fibres coated with a low friction material.
The tensile member may have a relatively high coefficient of fluid drag with the flowing fluid in the tube.
In a preferred embodiment, the tensile member is rotationally symmetrical.
This provides the advantage of minimising rotary movement of the tensile member, which may otherwise tend to bring the tensile member into contact with the wall of the tube, thus introducing frictional resistance.
According to another aspect of the invention, there is provided a method of inserting an elongate, flexible, light transmitting member into a plurality of elongate tubes arranged in series, the method comprising:
(a) inserting the light transmitting member into a first length of tube;
(b) feeding the forward end of the light transmitting member through a connector having a first inlet for sealingly receiving a forward end of the first length of tube, a first outlet for allowing escape of fluid flowing along said first length of tube, a second inlet for allowing entry of fluid flowing along a further length of tube, and a second outlet for sealingly receiving a rearward end of the further length of tube;
(c) connecting the forward end of the first length of tube to the first inlet and the rearward end of the further length of tube to the second outlet; and
(d) inserting the light transmitting member into the further length of tube by means of a method as defined above.
Because the light transmitting member is connected adjacent a forward end thereof to a tensile member during insertion into each length of tube, fluid flow in the further length of tube causes a forward tension to be applied to the light transmitting member in the connector, which overcomes any tendency for fluid flow in the first or further length of tube to cause the light transmitting member to move out of the connector in the rearward direction. This therefore provides the advantage that drive means is no longer required for inserting the light transmitting member into each length of tube at the input end thereof, with the result that installation speeds between successive drive means do not need to be coordinated, and damage to the light transmitting member is thus avoided.
In addition, because drive means are no longer necessary, the connector can have a much more compact and lightweight construction than prior art connectors.
The method preferably further comprises inserting the light transmitting member into a plurality of further lengths of tube by repeating steps (b) to (d) for each further length of tube.
In a preferred embodiment, a respective tensile member is inserted into each length of tube prior to positioning of the tube at the location at which the light transmitting member is to be inserted therein, and the method further comprises the step of detaching the tensile member of the first length of tube from the light transmitting member after installation thereof in the first length of tube, and attaching the tensile member of the or each further length of tube to the light transmitting member subsequently to feeding of the forward end thereof through the or each connector.
The or each connector may comprise at least two parts to facilitate removal thereof from the first and further lengths of tube.
Alternatively, the method may comprise the step of sealing the first outlet and second inlet of the or each connector subsequently to insertion of the light transmitting member.
This provides the advantage of preventing penetration of moisture or dirt into the or each connector.
The or each connector preferably further comprises an aperture so sized as to permit movement of the light transmitting member from the first inlet to the second outlet, but not passage of fluid from the second inlet to the first outlet.
In a preferred embodiment, the tensile member has a weight per unit length less than that of the light transmitting member.
This has the advantage that the tensile member tends to move along the tube more rapidly than the light transmitting member, as a result of which the tensile/light transmitting member combination in the further length of tube has a tendency to move along the tube more rapidly than the light transmitting member in the first length of tube. This in turn provides the advantage of avoiding buckling of, and consequent damage to, the light transmitting member.
The light transmitting member preferably comprises one or more optical fibres.
A preferred embodiment of the invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings in which: